1. Field of the Invention
This invention relates to a method and apparatus for forming an optical element wherein a glass material softened by heating is subjected to press forming by a pair of upper and lower forming molds.
2. Description of the Prior Art
Recently, a method of processing an optical element, herein a glass material (glass blank) softened by heating and subjected to press forming by a pair of tipper and lower forming molds has attracted attention over the previous method, where a glass material is ground and polished.
An explanation will now be provided of a conventional press forming method with reference to FIG. 5. In this method, a glass material 15 has previously been provided in a shape close to that of a formed final product. When performing press forming, the central portions of the glass material 15 contact the forming surfaces of the upper and lower forming molds. The upper mold 11 is removed from the apparatus, which is located in a heating furnace, then the glass material 15 is placed on a lower mold 12 and the upper mold 11 is set on the glass material 15. The entire apparatus is heated in the heating furnace at a temperature corresponding to a viscosity of the glass material 15 with which the glass material 15 can be formed. In this heating operation, the glass material 15 is heated by heat conduction from the upper and lower molds 11 and 12, and heat radiation from a cylindrical mold 13. In this case, since the radii of curvature of the forming surfaces 11a and 12a are similar to the radii of curvature of the surfaces of the glass material 15, and because gaps between the forming surfaces and the surfaces of the glass material 15 are small, excellent heat conduction is provided. Hence, the temperatures of the respective surfaces of the glass material 15 being formed are close to the temperatures of the corresponding upper and lower molds 1 and 2 and cylindrical mold 3.
As another conventional example, a spherical glass material is press formed as shown in FIG. 6. Such a shape can often be used when the size of a formed product, serving as an optical element, is small. In this case, since the distances between the respective surfaces of the glass material 25 to be formed, and the corresponding forming surfaces of the forming molds are large, heat conduction to the spherical glass material 25 is inferior to heat conduction when a glass material having a shape close to that of a final formed product is used. Even in this case, however, if a cavity made by the molds comprises a closed space, the glass material 25 can be heated to a temperature wherein the glass material 25 can be subjected to press forming by heat conduction and heat radiation from the molds.
However, the above-described method wherein a glass material having a shape close to that of a mold is used has the disadvantage of a high processing cost because of the costly glass material used. On the other hand, the use of a spherical blank can reduce the cost of the glass material to less than one tenth. Hence, it is desirable to perform press forming from a spherical glass material or a less expensive glass gob (a glass block obtained by cutting a fused glass stream) irrespective of the size of a formed product.
Recently, in order to shorten the forming cycle, apparatuses have been developed that place a glass material within a high-temperature forming mold that is located in an atmospheric furnace of a forming machine by using an automatic hand after press forming, a formed product is taken out from within the forming mold by the automatic hand, while the product is still at a high temperature of about 500 degrees. In this case, as shown in FIG. 7, it is necessary to provide an opening 34 for receiving an automatic hand 39 in a cylindrical mold 33. The presence of the opening 34 reduces heat radiation from the cylindrical mold 33 to the glass material, and obstructs the temperature rise of the glass material because heat is dissipated from the glass material through the opening 4.
As described above, it is preferable to use a spherical glass material and to handle the material with an automatic hand from the viewpoint of cost reduction. Such an approach, however, has the above-described problems. The problems will be sequentially explained with reference to FIG. 8. Since the heat transfer from the forming molds to a spherical glass material 25 is poor and because heat is radiated from the surface of the glass material 25, the temperature rise of the surface of the glass material 25 is significantly impeded. If press forming is performed in this state, the glass material 25 is deformed only slightly because the surface temperature of the glass material 25 is low. The contact areas between the glass material 25 and the forming molds increase due to this slight deformation, whereby heat conduction is increased. Hence, the surface temperature of the glass material 25 increases at the contact portion, causing a slight acceleration in deformation. Although it is possible to perform press forming by repeating the above-described process, a long time is needed for forming, particularly when a large glass material is used. Hence, such an approach is not practical. Furthermore, if press forming of such a low-temperature glass material is repeatedly performed, central portions of the forming surfaces of the forming molds, where high precision is required, are in some cases deformed. In order to provide a glass temperature suitable for forming, the temperature of the forming mold may be increased. In this case, however, since the temperature of the forming molds becomes higher than necessary, fusion may occur at the contact portion between the glass material and the forming molds during forming.